DESCRIPTION: The central question in project 3 is the extent to which doublestrand break repair (DSBR) is involved in genomic instability, with a focus on triplet repeat instability. The central axis of the project is FEN-1, the Okazaki fragment processing enzyme. The enzyme is hypothesized to be unable to process displaced 5 ends of Okazaki fragments ( flaps ) with secondary structures like hairpins, which might be a mechanism of triplet repeat expansion. This hypothesis also plays a dominant role in project 2 and FEN-1 is also studied as an auxiliary factor of slippage-mediated repeat expansion in project 1. In project 3, however, FEN-1 is hypothesized to play a more general role in genome instability by its proposed function in DSBR and even mismatch repair. Project 3 is also focused on a possible causal role of genomic instabilities in aging. The first two specific aims involve the generation of a FEN-1 knock-out and a FEN-1 tetracycline regulatable mouse model. These two aims are essential for this project, but also for project 2. Aim 3B and C of project 3 overlap with aim 1 of project 2. In aim 3 the FEN-1 mutants are studied for various forms of genetic instability and repair enzyme activity. Aims 4 and 5 continue this research line by studying chromosome instability by cytogenetic methods in the FEN-1 mutants in relation to histopathology as a function of age. Finally, in aim 6 the DNA-PK mutant defective in non-homologous end joining (NHEJ) will be crossed with Dr. Reddy s CTG repeat DM-PK mice to directly study the effect of defective double-strand break repair on triplet repeat expansion.